1. Field of the Invention
The present invention relates to an electrophoretic display device configured to implement display on the basis of migration of charged particles in a dispersing fluid.
2. Related Background Art
In recent years, proposals have been made on various types of electrophoretic display devices configured to implement display on the basis of migration of charged particles in a dispersing fluid. The electrophoretic display device will be described below.
With development of information equipment, there are increasing needs for low-power-consumption and low-profile display devices, and research and development is active on the display device to meet these needs. Among others, the liquid crystal display devices are actively developed and commercially available as display devices capable of meeting such needs. The current liquid crystal display devices, however, have the problem that characters on a display panel become harder to see, depending upon angles of view to the display panel and upon reflected light, and the problem that strain is heavy on the eyes because of flickering of the light source, low luminance, and so on. These problems have not been adequately solved yet. For this reason, the reflective display device is raising hopes in terms of low power consumption, reduction of the strain on the eyes, and so on.
Harold D. Lees et al. proposed the electrophoretic display device as one of such devices (U.S. Pat. No. 3,612,758).
FIG. 6A is a view showing an example of the structure of the electrophoretic display device, in which the electrophoretic display device of this type is composed of a pair of substrates 61, 62 placed with a predetermined clearance between; a dispersing fluid 63 filled between these substrates 61, 62; a number of colored, charged particles 64 dispersed in the dispersing fluid 63; and display electrodes 65, 66 positioned in each pixel so as to extend along the respective substrates 61, 62.
A cell wall 67 is placed between pixels to prevent the colored, charged particles 64 from migrating to the other pixels, thereby maintaining uniform display. In this device, because the colored, charged particles 64 are positively or negatively charged, either one display electrode 65 or 66 adsorbs the particles according to polarities of a voltage applied to the display electrodes 65, 66. Since the dispersing fluid 63 and the colored, charged particles 64 are colored in their respective colors different from each other, an observer visually observes the color of the particles 64 in a state in which the colored, charged particles 64 adsorb on the observer-side display electrode 65 (cf. FIG. 6B), but the observer visually observes the color of the dispersing fluid 63 in a state in which the colored, charged particles 64 adsorb on the other-side display electrode 66 (cf. FIG. 6A). Therefore, the device is able to provide display of various images on the basis of control on the polarities of the applied voltage in every pixel. Hereinafter, the device of this type will be referred to as a “vertical migration type”.
In the electrophoretic device of this vertical migration type, however, the dispersing fluid 63 has to contain a color coupler such as a dye, ions, or the like, and the existence of this color coupler induces new transfer of charge. The color coupler is thus likely to act as a destabilizing factor in the electrophoretic operation and sometimes degraded the performance, lifetime, and stability of the display device.
As a solution to this problem, Japanese Patent Application Laid-Open No. 9-211499 discloses an electrophoretic display device of the type shown in FIG. 7. The electrophoretic display device is composed of a pair of substrates 71, 72 placed with a predetermined clearance between; a dispersing fluid 73 filled between these substrates 71, 72; a number of colored, charged particles 74 dispersed in the dispersing fluid 73; and two electrodes 75, 76 placed in each pixel. The pair of electrodes 75, 76 are not placed so as to sandwich the dispersing fluid 73 between, different from the aforementioned type. The first electrode 75 is placed so as to extend along the rear-side substrate 72, while the second electrode 76 is formed in a portion covered by a shielding layer 77 provided on the front substrate 71.
In the case of the electrophoretic display device of this type, the dispersing fluid 73 can be transparent and does not have to contain a color coupler, so that the problem discussed above can be circumvented. In the device, the first electrode 75, or the surface thereof, or a color layer, which must be placed behind the first electrode 75 if the first electrode 75 is transparent, is colored in a color different from that of the charged particles 74. The charged particles 74 migrate according to the polarities of the voltage applied to the electrodes 75, 76. While the particles adsorb on the second electrode 76 below the shielding layer 77, the particles 74 are hidden under the shielding layer 77 from the observer, so that the observer does not observe the color of the charged particles 74 but visually observes the colors of the first electrode 75 and the shielding layer 77. While the charged particles 74 adsorb on the first electrode 75 on the other hand, the observer visually observes the color of the charged particles 74 and the color of the shielding layer 77. Accordingly, the device is able to provide display of an image on the basis of control on the polarities of the applied voltage in every pixel.
The electrophoretic display device described above, however, had the following problems. In Japanese Patent Application Laid-Open No. 9-211499, the first electrode 75 is provided in flat shape on the rear-side substrate 72, and the second electrode 76 is structured to be placed under the peripheral shielding layer 77. In this configuration, when an electric field is impressed between the two electrodes 75, 76 in order to move the charged particles 74, as apparent from FIG. 7, the electric field is concentrated at the part closest to the second electrode 76, in the surface of the first electrode 75, i.e., at the edge of the surface of the first electrode 75. This raised the problem that, for example, in an operation of moving the charged particles 74 present on the second electrode 76 onto the first electrode 75, many charged particles 74 migrated to the edge of the first electrode 75 and it was hard for the particles to reach the central region, thus lowering the display contrast.
Furthermore, as shown in FIG. 8, the above electrophoretic display device has relatively large viewing angle dependence of the positional relation between the display layer formed on the first electrode and the shielding layer formed on the observer side of the display-side substrate. This viewing angle dependence occurs where two optical modulation layers are placed with a spacing over a definite distance in the in-depth direction, and poses problems of decrease of aperture ratio, color shift, parallax, and so on. The above conventional example also had the problem that when the observer obliquely observed the pixel, the shielding layer was projected in a large area on the surface of the first electrode to decrease the effective aperture ratio and particles on the side wall appeared exposed in part to cause mixing of colors.
Since this situation varies depending upon angles of observation, the display inevitably has great viewing angle dependence as a result. The viewing angle dependence becomes noticeable, particularly, in high definition display, and its influence becomes serious, for example, in the case of the pixel pitch of 100 μm; even if the clearance between the upper and lower substrates is controlled at 20 μm, at least the display-side substrate needs to be a plastic substrate 50 to 100 μm or more thick.
In the above application, since the shielding layer 77 is formed on the observer-side surface of the display-side substrate 71, the substrate must have some thickness, which posed the problem that each pixel display covered by the shielding layer 77 varied depending upon view.